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Interface Modification of Nylon 6/NBRComposites through Bonding Agents
Rajesh C. “Development and characterisation of Nylon Fibre Reinforced NBR composites” Thesis. Department of Chemistry, University of Calicut, 2007
Chapter 4
Interface Modification of Nylon 61NBR
Composites through Bonding Agents
Abstract
The interface modfication of short nylon b j b r e reinforced NBR composites has been
carried out by the incorporation of two dzflerent bonding agents, viz., hexamethylene
tetramine-resorcinol and phthalic anhydride. The cure characteristics and mechanical
properties of the bonded composites have been analysed. The addition of bonding
agents increases the maximum torque while reducing the optimum cure time and scorch
time of the composites. The bonding agent added composites exhibit superior
mechanical properties than the corresponding unbonded systems. The improved
adhesion between fibres and rubber in bonding agent added composites has been
conJirmed by scanning electron microscopy.
Chapter 4: Interface ModiJication through Bonding Agents
4.1 INTRODUCTION
The properties of fibre reinforced composite materials depend on the combined
behaviour of the reinforcing fibres, polymeric matrix and the fibre-matrix interface.
Existence of good interfacial bonding between fibres and matrix is an essential
requirement to achieve good physical properties. In composites, the matrix
molecules may be anchored to the fibre surface by chemical reactions or adsorption
which determines the extent of interfacial adhesion. Interaction at the interface can
be improved either by using a bonding agent or by different surface treatments for
the fibre. These additives can modify the interface by interacting either chemically
or physically with both the fibres and the matrix'.
Several researchers have reported the use of different bonding agents to modify the
interfacial strength of fibre-rubber composites. A tricomponent dry bonding system
based on hydrated silica-resorcinol-hexamethylene tetramine (HRH) has been used
successfully in many short fibre elastomer ~ewi t t ' ) has
demonstrated the importance of zinc oxide and stearic acid in the HRH system to
promote fibre-matrix adhesion. ~ u n n o n ' ~ proposed the rough guidelines for the
manipulation of the compounding and processing factors in the course of designing
an optimum adhesion compound. Darwish and ~ a w a n d ~ ' ~ reported the use of HRH
as an adhesion promoter for nylon cord to NBR. They also studied the effect of
replacement of resorcinol with o-aminophenol, m-arninophenol, p-aminophenol and
m-phenylene diamine. Pokluda and 0sohaI6 used different types of cresols to
replace resorcinol in HRH bonding system. Varghese et all7 showed that silica
Chapter 4: Interface Modification through Bonding Agents
was not needed as a component in the dry bonding system when sisal fibre was used
as reinforcement. The effects of urethane resins based on polymeric 4, 4'-
diphenylmethane diisocyanate (MDI) and different diols on the mechanical
properties of short polyester fibre reinforced polyurethane composite were studied
18, 19 by Suhara et a1 . The urethane based bonding system affected the processability
of the composites adversely. However, the tensile strength, modulus, and the
abrasion resistance of the composites were improved by the urethane bonding
system. Sreeja and investigated the effect of a bonding agent, based on
urethane, on the cure characteristics and mechanical properties of NRlwhole tyre
reclaim-short nylon fibre composites. Haseena et ~ 1 . ~ ' reported the use of a two
component bonding system consisting of hexamethylene tetramine and resorcinol in
short sisallcoir hybrid fibre reinforced NR composites.
The interface modification of short nylon 6 fibre reinforced NBR composites by
using two different bonding systems, namely hexamethylene tetramine- resorcinol
and phthalic anhydride is the subject matter of this chapter. The efficiency of these
bonding agents has been evaluated in terms of cure characteristics and mechanical
properties.
4.2 RESULTS AND DISCUSSION
4.2.1 Curing behaviour
The addition bonding agents has a considerable effect on the cure characteristics of
short nylon 6 fibre reinforced NBR composites. Figure 4.1 represents the typical
rheographs of mixes containing hexa-resorcinol and phthalic anhydride bonding
Chapter 4: Interface Modification through Bonding Agents
agents (Mixes Q and R) and that of the mix without bonding agent (Mix M), cured
by DCP containing 24 phr nylon fibres of length 6 mm.
Time (min)
Figure 4.1 Rheographs of mixes M, Q and R
Table 4.1 shows the cure characteristics of sulphur and DCP vulcanised composites
containing different bonding agents along with the corresponding unbonded ones. It
can be seen that in both sulphur and DCP cured samples the torque values of the
mixes containing bonding agents are higher than those of mix without bonding
agent. The difference between the maximum and minimum torque value (MH-ML)
indicates a higher crosslinking density for the bonding agent added composites. In
the case of resorcinol-hexa bonding agent added mixes, the higher crosslink density
can be attributed to the increased adhesion between the fibre and matrix through the
formation of an in-situ resin. During the curing of the composite, polymerisation of
resorcinol and hexamethylene tetramine is initiated and, as the vulcanisation of
Chapter 4: Interface Modification through Bonding Agents
rubber proceeds, the resin is formed by the condensation reaction between these two
components. Meanwhile, the low molecular weight polymer species of the resin are
able to diffuse into the interfacial region between the rubber and the fibre. Thus, a
boundary layer is formed at the rubber surface which is rich in resin formation and
it forms chemical bonds with the rubber and the fibre. Scheme 4.1 represents a
hypothetical mechanism showing the interface bonding in hexa-resorcinol bonded
composites.
Table 4.1 Cure characteristics of various mixes
In the case of phthalic anhydride added mix the increased crosslinking density can
be due to the improved adhesion between fibres and matrix through intermolecular
hydrogen bonding (Scheme 4.2). It is also found that the addition of bonding agent
reduces the optimum cure time and scorch time (Table 4.1). The decrease in scorch
time, however, indicates a reduction in scorch safety of the compound containing
bonding agents.
Chapter 4: Interface Modification throuxh Bonding Agents
OH
OH + ~J-- -HN-(CH~)~--CV
Nylon 6 fibre
CH*'" CH2
Resorcinol
Hexamethylene tetramine +
NBR
Nylon 6 Fibre
Resin
NBR
Scheme 4.1 Mechanism of interfacial adhesion in hexa-resorcinol bonded composites
Chapter 4: Interface Modification through Bonding Agents
Nylon 6 fibre
NBR
R
@) +
0
Phthalic anhydride
CN I
W- CH~-CH=CH--CH~-CH~--€H-~
l Nylon 6 fibre
r H N - - ( C ~ 2 ) 4 0 - u l
,
0 l I
@Oo 0 I I
I
,
C=NH I I
CH&H=CH-CH&H2--C_fLr NBR
Scheme 4.2 Mechanism of interfacial adhesion in phthalic anhydride bonded composites
4.2.2 Mechanical Properties
Figure 4.2 shows the stress-strain curves of the unbonded and bonded composite
systems cured by DCP. The effects of different bonding agents on the mechanical
properties of the longitudinally oriented composites can be understood from
Chapter 4: Interface Mod$cation through Bonding Agents
0 0 10 20 30 40 50
Strain (%)
Figure 4.2 Stress-strain curves of bonded and unbonded composites
Table 4.2. It can be seen that composites containing bonding agents show superior
mechanical properties than those without bonding agents. Tensile strength, tear
strength, modulus and hardness of the composites have been found to be increased
by the incorporation of bonding agents, whereas the elongation at break to be
reduced. The improvement in tensile strength and modulus is comparatively higher
for the hexa-resorcinol bonded composites than the phthalic anhydride bonded
systems. This can be attributed to the difference in the extent of interfacial
interaction between the fibres and rubber resulted via the incorporation of bonding
agents. It is also obvious from Table 4.2 that the effect of hexa-resorcinol boding
agent is more pronounced in DCP cured composites than in the sulphur cured one
(Mixes Q and G respectively). This can be attributed to the difference in the type of
crosslinks introduced by DCP and sulphur during vulcanisation which has been
explained earlier (Chapter 3, Section 3.2.2.3).
The extent of W a l & W o n has been better d e m t d by examining the
Rgum 43 (a) Scanning d&mn micrograph of the ten& ififare surface of mix M
(b) Scanning d e n micrograph offhe temiIe hiham surfme of mlr Q
Chapter 4: Interface Modzjication through Bonding Agents
4.3. CONCLUSIONS
Hexamethylene tetramine- resorcinol and phthalic anhydride were used as bonding
agents to modify the interfacial interaction in nylon 6 fibre reinforced NBR
composites. The cure characteristics and mechanical properties of the bonded
composites have been analysed. The addition of bonding agents increased the
maximum torque and reduced the optimum cure time and scorch time. The addition
of bonding agents enhanced the tensile strength and modulus. SEM studies revealed
improved adhesion between the fibre and rubber in the composites. The effects of
these bonding agents on the properties of the composites have been included in the
subsequent chapters.
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